I can’t take it anymore :(

This is my 4th baby but I’ve never felt this type of exhaustion with any of my others. I keep falling asleep every hour or so and I have no energy to do anything. I can’t even clean or cook how I used to because of this fatigue. I feel so shittyyyy everyday. And at night I can’t sleep at ALL I keep waking up every 15 mins or having pain in my leg / hip. I’m only 6 weeks. I heard everyone says take magnesium glycinate. Does the OB have to prescribe it ? If not, can anyone take me good brands that actually makes a difference?